just use dragon and attack is have high power base, and also if you have 3 gold star it will help you can 1-hit. and also using weakness of octopus dragon it will make huge chance for 1-hit.

lets see, maybe it will help you out.

dark, is weak to terra and nature. and your dragon must not weak to sea or dark (your dragon must not sea, flame, electric, metal on first type).

Rottening spell, Bee hive, Asteroid, Sand storm, Tumble weed attack it will help you a lot than other type of attack. but attack from training is have minor effect to non-trainer attack so you need think before pick a attack on training list, bold one is recommend one since is have high power base than Rottening spell/Asteroid/Meteor Shower.

Let's see on good move for that (is just example only, i not going to give all in here since it will long XD) :

Tumble weed : Cactus Dragon, Chameleon Dragon,Snowflake Dragon (training-2200), Armadillo Dragon (non-training-2750), so Armadillo dragon is good idea for you used Tumble weed move (also first type is not metal so is OK), if you have pure terra, is perfect choice since is higher stat, but cactus and snowflake is good choice too.

Rottening spell : Dragonfly Dragon (training-2200) , Gummy Dragon (training-2750). Dragonfly dragon is good choice too but gummy dragon is better you can pick to attack octopus dragon. Pure dragon is another recommend on this part.

Bee hive : Coral Dragon and Cactus dragon (training-2200). Is good idea too for using Bee hive, but don't using nature dragon, is too weak XD, coral and cactus is better option.

I know is all need time to train that move, if you want non training attack, just stick to armadillo dragon.

I think a dragon with plant element is the one you should use. I used my tropical dragon and it got critical with earth attacks as well. Too bad it wasn't 'critical' enough to defeat the Octopus dragon.